What property of real numbers is 3 + ((-5) + 6 = (3 + (-5)) + 6

The associative property

Step-by-step explanation:

I assume you meant 3 + ((-5) + 6) = (3 + (-5)) + 6.

This is the associative property of addition which states:

a + (b + c) = (a + b) + c

This property also works with multiplication.